The present invention relates to solid state image sensors, and more particularly to solid state image sensors for reproducing high definition images.
Charge transfer devices (CTD) such as charge coupled devices have been known as solid state image sensors adaptable for standard television system, for example, National Television Systems Committee (NTSC) system and have found a variety of applications. In NTSC system, the number of vertical scanning lines is 512, the scanning system is of an interlacing scanning type with two fields for one frame, and the aspect ratio is 3:4. The number of picture elements of a CCD adaptable for the standard TV system, for example, in interline transfer type CCD (IT-CCD) is approximately 500 (vertical) .times.400 (horizontal).
The solid state image sensor such as the IT-CCD is superior to the conventional image pick-up tube in many points. For example, the image sensor is small in size, light in weight, and high in reliability. Further, it is essentially free from pattern distortion and sticking, and is little influenced by afterimage sensor. Having such excellent features provide a prospect of increasing applications in many fields. Example, ITV, small commercial video cameras which will supercede the conventional cameras using silver salt-film. For applying the image sensor for such fields, a remarkable resolution improvement of the reproduced picture is required for the solid state image sensor in order to ensure a high quality of picture.
High definition wide television systems, which can provide wide and high definition pictures are reagrded as the next generation television systems. The study of the high definition wide television system reports that the scanning lines must be 1,000 or more, for example, 1,125. To satisfy such requirement, the improvement of the resolution of the solid state image sensor is very significant.
The number of picture elements 500.times.400 of the currently used solid state image sensor, for example, IT-CCD, is too small to realize the high resolution. Therefore, it is considered that the number of picture elements must be remarkably increased. It is to be noted that the solid state image sensor currently used needs one of the largest chip sizes of LSIs. Therefore, if the number of picture elements is simply increased for the above purposes, the chip size of the image sensor is considerably increased. This leads to increase of the size and cost of the image sensor. On the other hand, if the number of picture elements is increased while keeping the present chip size of the image sensor, the integration density of the chip must be improved considerably, for example, four times or more. The LSI chips with such a high integration density, however, involve a difficulty in fabricating them by the presently existing fabricating technology. Even if in the future, the fabricating technology advances and realizes the solid state image sensors with a high integration density, a new problem arises that a drive system for such image sensors must be complicated and consume large power.